diff --git a/tasksetgen.py b/tasksetgen.py
index 1b8c702..75330bc 100755
--- a/tasksetgen.py
+++ b/tasksetgen.py
@@ -46,6 +46,8 @@ def query_yes_no(question, default=None):
             return False
 
 def gcd(a, b):
+    if b == 0:
+        return a
     return gcd(b, a % b)
 
 def lcm(a, b):